The invention concerns a position sensor which detects the position of a very small mirror, and provides a feedback signal indicative of the position.
FIG. 1 illustrates a small mirror 3, of 500 microns diameter. Since 25.4 microns are contained in one mil, which is {fraction (1/1,000)} inch, the diameter is equivalent to (500/25.4) mils, which equals about 20 mils. As two points of reference, (1) an ordinary sheet of typing paper is about four mils in thickness, and (2) the head of a straight pin, used by tailors, is about 80 mils in diameter.
The mirror is front-surface type, bearing a gold coating (not shown).
This type of mirror is used in Micro Electro Mechanical Switches, MEMS. FIG. 2 is a simplified schematic of part of an MEMS. Mirror 3 is shown, together with photodetectors 6, and a laser 9. The laser 9 projects an input signal 10 to the mirror 3. The mirror 3 is driven to a rotational position, indicated by phantom mirror 3P, by electric fields which are not shown, and reflects the laser signal to photodetector 6A, which delivers the signal to an output line, not shown.
In practice, feedback is provided, as shown in FIG. 3, to assure that the laser signal reaches the intended photodetector. For example, a second laser 12 projects a second laser beam 15, of different frequency, which is also reflected by mirror 3. A positioning detector 18 is associated with each photodetector 9. Receipt of the second laser beam 15 by the appropriate positioning detector confirms that the intended photodetector receives the laser signal.
FIGS. 2 and 3 are simplifications. Actual MEMS contain two-dimensional arrays of photodetectors, and the arrays can contain 256, or more, photodetectors. This type of MEMS is known in the art, and is described, for example, in the publication R and D, July, 1999, page 35.
One disadvantage of this type of MEMS is that the feedback system of FIG. 3 is complex and expensive.
In one form of the invention, strain gauges are used to support the mirror. The rotational position of the mirror changes the electrical resistance of the strain gauges, thereby producing a feedback signal indicative of mirror position.